1. Field of the Invention
The invention relates to mounts for filtration elements such as a mount provided by the facepiece of a respiratory mask. The invention specifically concerns a respiratory mask having a porous, rigid or semi-rigid filtration element or elements for protecting the wearer against hazardous gases and vapors. The invention also concerns a method of making a filtering device using an injection mold adapted to receive as an insert a porous, rigid or semi-rigid filtration element. By "semi-rigid" is meant that the filtration element is shape-retaining and can be moderately flexed by one's fingers.
2. Description of the Related Art
A respirator mask should have a soft, supple facepiece to permit the mask to be worn for prolonged periods without undue discomfort. When the respirator mask employs replaceable filters, the filter typically consists of a filtration element mounted in a casing which is threadably received into a complementary casing of the facepiece. Hence, there is the possibility of leakage between the filtration element and its casing, between the two casings, and between the complementary casing and the facepiece. Especially worrisome is the possibility of leakage between the two casings, because some users may be careless when replacing filters. Another problem with prior respiratory masks is that the filters often project from the facepiece to a degree interfering with the user's actions as well as with the user's vision. Such a respirator mask is shown in Maryyanek et al., U.S. Pat. No. 4,592,350.
After discussing problems with respirator masks of the replaceable filter type, Witman et al. U.S. Pat. No. 3,861,381 describes injection molding the shell or facepiece of a respirator mask which protects the wearer against particulate matter. The shell material is made to flow around individual fibers of a flexible, fibrous filtration element to provide a mechanical joining of the shell and filter. "The mold which shapes the shell and holds the filter during molding is provided with compression areas which pinch off the major portion of the filter while allowing the material of the shell to penetrate a predetermined distance into the filter thereby encapsulating and sealing the edges of the filter and joining it to the shell. Support pins may be used in the mold to locate the filter prior to the molding operation" (col. 2, lines 32-39). A "continuous circumferential portion of the filter in the vicinity of its periphery is compressed as at 14, to the point where it is no longer porous to the melted plastic, thus preventing the penetration of shell material into the working area of the filter during the molding operation" (col. 3, lines 44-51). The melted plastic flows into the interstices between the uncompressed fibers outside the line of compression and solidifies around those fibers while either melting those fibers or otherwise becoming bonded to the periphery of the filtration element.
The respirator mask illustrated in the Witman patent should be far less expensive to make than are respirator masks that have replaceable filters in that the respirator mask is assembled as the facepiece is formed.
A method of making a facepiece of a respiratory mask that is similar to that of the Witman patent is described in Bradley et al. U.S. Pat. No. 2,922,417. Like Witman, Bradley does not suggest the use of injection molding until after the edges of the filtration element have been sealed. In the Bradley patent, the sealing can be done by spraying adhesive onto the uncompressed edges of the filtration element while a continuous circumferential portion of the filter in the vicinity of its periphery is compressed to the point where it is not porous to the adhesive.
Boylan U.S. Pat. No. 3,183,285 concerns another method for creating a mount or gasket for a filtration element. After pouring an expandable plastic compound into a peripheral channel of a mold, the edges of the filtration element are moved into contact with the plastic compound. The upper mold member is closed and clamped, both closing the channel and pinching the filtration element along the line just inside the periphery of the filtration element. Then the plastic compound is allowed to expand to the limits of the closed channel, thus impregnating that portion of the filtration element that extends into the channel. Holloway et al. U.S. Pat. No. 3,235,633 concerns a method similar to that of the Boylan patent except that nothing is said about closing the mold into which the expandable plastic is poured.
3. Other Prior Art
Preferred semi-rigid filtration elements for use in the respiratory mask of the present invention are bonded absorbent granules as disclosed in Braun et al. European Patent Application No. EP-218348A (published April 15, 1987), which disclosure is here incorporated by reference. The filtration elements provide protection against gas and vapors and can also provide protection against particulate matter.
U.S. Pat. No. 4,664,683 (Degan et al.) also describes filtration elements useful in the respiratory mask of this invention.